The present invention is concerned with new crystalline copolymers of propylene, and with the process for preparing them.
Those skilled in the art know that polypropylene can be modified by introducing small amounts of comonomers, mainly ethylene and 1-butene, during the polymerization reaction. The purpose of this modification is of lowering the melting point of the polymer, and hence obtaining films showing improved characteristics of weldability.
However, the polymers obtained by means of the processes known to date suffer from the drawback of a non-homogeneous distribution of the comonomers; this causes the polymers to show high solubility characteristics in cold xylene, and the manufactured articles obtained from them to have decreased mechanical properties.
From European patent application N. 0 185 918, polymerizing propylene with stereospecifc catalysts is known, wherein such stereospecific catalysts are obtained from stereorigid and chiral compounds of zirconium, such as ethylene-bis-indenyl-zirconium dichloride, and ethylene-bis(4,5,6,7-tetrahydroindenyl)zirconium dichloride, and from methylalumoxanic compounds.
The catalysts are also used in the polymerization of mixtures of propylene with ethylene or other olefins, with copolymers being obtained which, in the examples, are rich of ethylene and are soluble in xylene, and show an isotactic configuration of the propylenic sequences.
The present Applicants have unexpectedly found now that by using catalytic systems obtained from:
a) a sterorigid and chiral compound of zirconium of formula 
xe2x80x83wherein:
R1 and R2 are halogens or alkyl groups of from 1 to 6 C atoms;
X and Y are asymmetrical single-ring or multi-ring organic groups;
R3 is a linear group of from 1 to 4 C atoms, or a cyclic group containing from 3 to 6 C atoms;
b) an alumoxanic compound of formula 
xe2x80x83with n being comprised within the range of from 2 to 25; or 
xe2x80x83with n being comprised within the range of from 1 to 25; propylene copolymers with minor amounts of ethylene and/or alpha-olefins
CH2=CHR
wherein R is an alkyl radical of from 2 to 20 C atoms, which are endowed with a high crystallinity, and with very good mechanical properties (very similar to propylene homopolymer), have a melting point comprised within the range of from 110xc2x0 to 140xc2x0 C. and show a limited solubility in (soluble polymer fraction in xylene at 25xc2x0 C. lower than 10%) cold xylene/, can be obtained if the polymerization of the monomers is carried out under conditions under which the formed copolymer has a composition similar to the composition of the monomers present in the gas phase.
The present Applicants have found, and this is an at all unexpected aspect of the present invention, that the composition of the copolymers is similar to that of the mixture of the monomers present in the gas phase, when the content of the monomers which are not propylene of said mixture is comprised within the range of from 2 to 10 mol %.
More particularly, in case of copolymers with ethylene, the ethylene content in the gas-phase mixture is comprised within the range of from 2 to 6 mol %; in case of propylene-ethylene-butene terpolymers, the content of ethylene and butene is comprised within the range of from 2 to 10 mol %.
By operating under the above stated conditions, copolymers are obtained, in which the distribution of the comonomers is perfectly homogeneous. This is demonstrated, in case of the copolymers with ethylene, by the analysis by means of 13C-N.M.R.: the presence is not observed of the characteristic signals of the sequences of more than one ethylene units, as described in Macromolecules 10 (3) 536 (1977).
The preparation of the copolymers by using the above stated catalysts is carried out by operating both in the gas phase and in the liquid phase (in the presence of a solvent consisting of an inert hydrocarbon, or in liquid propylene). The composition of the gas phase, comprised within the above stated critical range, is maintained constant during the polymerization. For example, in case of polymerization of propylene-ethylene mixtures by operating in liquid propylene, a constant overpressure of ethylene is maintained.
On the contrary, in case the polymerization is carried out in the gas phase, or in the presence of an inert hydrocarbon solvent, a gaseous mixture of the monomers with a constant composition is fed. The polymerization is carried out at temperatures lower than 20xc2x0 C., and preferably comprised within the range of from 0xc2x0 to 10xc2x0 C.
The copolymers obtained by operating under the above stated conditions have an intrinsic viscosity in tetralin at 135xc2x0 C. higher than 0.2 dl/g. As already stated, the copolymers are mainly used in the field of films; this, thanks to the high weldability characteristics of the films obtained from them.